1. Field of the Invention
The present invention relates to a room temperature curable polyorganosiloxane composition containing a specific calcium carbonate. This composition cures to a silicone elastomer which exhibits improved thermal stability properties and physical properties.
2. Description of the Prior Art
Polyorganosiloxane compositions that cure at room temperature by a condensation reaction are generally called RTV (Room Temperature Vulcanizing) silicone rubbers. They are widely used for various services and in various fields because of the ease of the curing reaction and the excellence of the electrical, chemical, and thermal properties of the cured material. Such RTV silicone rubbers are composed mainly of polyorganosiloxane (as the base), crosslinking agents, and fillers. Fillers such as dry silica, wet silica, diatomaceous earth, quartz powder, calcium carbonate, and alumina are generally used. The silicas are the most widely used fillers, and RTV silicone rubbers in which these are utilized are very strong, although the amount of the filler which can be added is restricted because the viscosity of the polyorganosiloxane compositions greatly increase as the amount of silica increases. The viscosity increase with quartz powder and alumina is slight, but the resulting strength is not very great.
Additionally, the use of calcium carbonate as a filler for RTV silicone rubbers is well known and has been presented, for example, in Japanese Patents [Tokkai] Sho No. 52[1977]-2439 and [Tokkai] Sho No. 53 [1978]-34855. Since the viscosity increase and the strength imparted by the calcium carbonate vary according to the various grades, such as heavy calcium carbonate, light calcium carbonate, and fine light calcium carbonate, they are best chosen in ways which are appropriate to their purposes and applications. However, conventional calcium carbonates have the drawback that when combined in RTV silicone rubbers, they have poor thermal stabilities above 150.degree. C.
Calcium carbonate has also been used in one package RTV silicone rubber compositions, such as those made from hydroxyl endblocked polydiorganosiloxanes, finely divided agglomerated precipitated acicular calcium carbonate, acyloxysilanes, and tin carboxylates. Such compositions are described as paintable by Clark and Johnson in U.S. Pat. No. 3,957,714, issued May 18, 1976, and by Smith in U.S. Pat. No. 4,247,445, issued Jan. 27, 1981, and U.S. Pat. No. 4,293,616, issued Oct. 6, 1981.